Java8 Stream 之归约操作

1. 归约方法

public interface Stream<T> extends BaseStream<T, Stream<T>> {

    Optional<T> reduce(BinaryOperator<T> accumulator);
    
    T reduce(T identity, BinaryOperator<T> accumulator);

    <U> U reduce(U identity, BiFunction<U, ? super T, U> accumulator, BinaryOperator<U> combiner);
}

2. collect() 的归约

• 归约操作：将流中的元素规约为单一值（通过重复应用二元运算符）

归约操作的工作原理：
利用累积函数，把一个初始化为起始值的累加器
把转换函数应用到流中每个元素上得到的结果不断迭代合并起来

3. Integer的归约

3.1 元素求和

reduce()对数字流求和

@Test
public void patternOne() {
    List<Integer> numbers = Arrays.asList(1, 2, 3);

    int sum = 0;
    for (int num : numbers) {
        sum += num;
    }

    Assertions.assertEquals(sum, 6);
}

@Test
public void patternTwo() {
    int count = Stream.of(1, 2, 3)
            .reduce(0, (acc, element) -> acc + element);

    Assertions.assertEquals(count, 6);
}

展开 reduce 操作

@Test
public void expandReduce() {
    BinaryOperator<Integer> accumulator = (acc, element) -> acc + element;
    int count = accumulator.apply(
            accumulator.apply(
                    accumulator.apply(0, 1),
                    2),
            3);
}

3.2 计算金额

@Test
public void test () {
    List<OrderDTO> list = Arrays.asList(new OrderDTO("1001", 10200L), new OrderDTO("1002", 200L));

    Optional<Long> orderAmounts = list.stream()
            .map(OrderDTO::getOrderAmount)
            .reduce((a, b) -> a + b);

    Long amount = list.stream()
            .map(OrderDTO::getOrderAmount)
            .reduce(Long::sum)
            .orElse(0L);

    System.out.println(amount);
}

3.3 最大值和最小值

@Test
public void maxReduce() {
    List<Integer> numbers = Arrays.asList(10, 25, 3);
    
    // Optional<Integer> optional = numbers.stream().reduce(Integer::max);
    Integer max = numbers.stream()
            .reduce(Integer::max)
            .orElse(0);
}

@Test
public void minReduce() {
    List<Integer> numbers = Arrays.asList(10, 25, 3);
    Integer min = numbers.stream().reduce(Integer::min)
            .orElse(0);
}

3.4 计算流中元素的个数

把流中每个元素都映射成数字 1，然后用 reduce 求和

@Test
public void count() {
    List<String> menus = Arrays.asList("chicken", "duck", "sausage");
    int count = menus.stream()
            .map(d -> 1)
            .reduce(0, (a, b) -> a + b);

    // or menus.size()
}

map 和 reduce 的连接通常称为 map-reduce 模式
也可用 count() 来计算个数 long count = menus.stream().count();